(1) Field of the Invention
This invention relates to electrical connector plugs, and more particularly to improved 3-wire, automatic grounding type connector plugs.
(2) Description of the Prior Art
The hazard of operating ungrounded electrical devices is well recognized. As a result, the Underwriter's Laboratories, Inc. and many electrical codes long have required that all portable electrical tools be equipped with suitable grounding means. In the beginning, this requirement was satisfied by the use of a commonly called "pig-tail" connection in the form of a wire coming out of one side of the plug, and which had been connected to one of the blades of the plug, terminating in a contact for attachment to a neutral ground lead, such as the box carrying the receptacle.
More recently, the "Underwriter's" have required that a plug for electrical devices be equipped with a three-prong plug for insertion into a 3-wire receptacle. Although this is a desirable objective from a safety standpoint, the fact is that many buildings where electrical devices might be used have only 2-wire electrical outlets. That, of course, presents a problem to manufacturers of portable electrical tools and other equipment. Moreover, when users of portable tools having a three-prong plug have encountered a 2-wire outlet, they have often reacted by breaking off the ground prong in order to use the electrical device. This, of course, not only destroys the usefulness of the 3-wire plug where a three-prong outlet is later encountered, but results in an unsafe condition.
To overcome the problem presented, i.e., how do you use a three-prong plug with a 2-wire receptacle, a variety of adapters or conversion plugs have been invented having only two blades. A three-prong plug is then plugged into the adapter which in turn is plugged into the 2-wire receptacle. One such adapter is provided with a "pig-tail" connected at one end to a ground connection in the adapter and terminating at the other end in a ground contact which can be connected to the ground screw on the 2-wire electrical outlet. While these adapters are satisfactory to a degree, in many instances a user does not have a screwdriver available in which to make the connection. And in many instances, the user of the portable tool may not even have an adapter readily available.
Over the years, a variety of three-prong plugs have been developed which make unnecessary the use of an adapter, as above-described. Examples of prior art patents disclosing various convertible three-prong plugs are U.S. Pat. Nos. 2,323,736; 2,922,134; 2,986,718; 2,876,426; 3,134,631; 3,178,667; 3,219,962; and 2,984,808.
As disclosed in U.S. Pat. No. 2,323,736, there is shown a three-prong plug in which the ground prong is capable of being retracted from a position of use, depending on whether or not a 3-wire electrical outlet is encountered. When retracted for use with a 2-wire outlet, however, this leaves the electrical device with which the plug is associated ungrounded. U.S. Pat. No. 2,984,808 discloses another three-prong convertible plug in which the ground plug can be pivoted into the use position or not as desired. A "pig-tail" connection is provided on the plug, and in connection with the ground, for connection to the ground screw of a 2-wire receptacle. Another three-prong plug in which a pivotable ground prong is provided is disclosed in U.S. Pat. No. 2,986,718. A "pig-tail" is provided which connects to the ground and which is provided with a connection at the other end for connection to the ground screw in case of use with a 2-wire electrical outlet.
U.S. Pat. Nos. 2,876,426 and 2,922,134 disclose electrical connecting plugs in which a pivotable grounding element is provided. The ground prong on encountering the 2-wire outlet is pivoted so that the finger portion of the grounding prong is out of position so that the connecting plug can be used. In U.S. Pat. No. 2,876,426, the other end of the pivoted prong is constructed so that on the prong being pivoted, it is in contact with the grounded receptacle plate and, in turn, grounds the electrical device.
U.S. Pat. No. 3,134,631 discloses another three-prong electrical plug in which the ground prong is pivotable out of position in case the plug is used with a 2-wire electrical outlet. The ground prong is provided with a groove adjacent its free end for insertion of a ground wire which in turn can be connected to a ground lead in the electrical outlet. A further electrical plug in which is provided a pivotable prong is disclosed in U.S. Pat. No. 3,178,667. When a 2-wire outlet is encountered, the pivotable ground prong therein is resiliently urged against the face plate of the receptacle to establish a ground, thereby reducing the possibility of a shock being transmitted to the user of an electrical device.
U.S. Pat. No. 3,219,962 discloses an electrical connector plug in which a pivotable ground prong is provided and which is disclosed for use with either a 2 or 3-wire receptacle. The ground plug is biased to its operative position.
While all of the electrical plugs as above disclosed are suitable to a degree, their use does involve certain problems and disadvantages. In some cases, no ground is provided for at all, making for an unsafe condition. Where a "pig-tail" is used to provide connection to a ground screw, this necessitates removal or at least loosening a receptacle ground screw. However, many times a screwdriver is not readily available. In any event, it makes use of a ground less likely because grounding is not readily accomplished without the user taking some further action other than merely pushing the plug into the outlet. Where grounding is to be accomplished by a pivotable ground prong making contact with an electrical receptacle face plate, certain mechanical problems sometimes result. The biasing mechanism sometimes, particularly after use for some time, does not properly bias the ground prong against the face plate, thereby providing no ground.